Vehicle drive-trains of mobile working machines, in particular wheel loaders, are more and more often made with continuously power-split transmission devices, usually equipped with a variator, a reversing transmission and range clutches. In the area of the transmission unit associated with the variator, spur gear stages or planetary stages are usually provided. Driving ranges within which a transmission ratio can be varied continuously by the variator are provided by the transmission unit. To carry out driving range changes, as a rule disk clutches are used as shifting elements. Moreover, the variator is often designed as a hydrostatic transmission with a hydrostatic unit consisting of a pump and a motor.
To change the transmission ratio of such a transmission device, for example starting from small transmission ratios toward larger transmission ratios, in each case one or more driving ranges are passed through and for this the hydrostatic unit correspondingly has to be swiveled across the full driving range several times. For example, if two complete driving ranges have to be passed through, then within a first driving range the hydrostatic unit operates to adjust the transmission ratio of the transmission unit continuously until the end of the driving range. Thereafter, two controlled range clutches coordinated with one another carry out a driving range change to the second driving range, which has a subsequent transmission ratio band, which is also passed through by means of the hydrostatic unit until the maximum transmission ratio has been reached.
When a wheel loader drives up a mound, the additional external load imposed on the drive-train, particularly the deceleration, increases in less than 0.5 seconds. The control rate or speed of change of the transmission ratio in the area of the transmission device is therefore disadvantageously too low to avoid unacceptably large falls of the rotational speed of the drive engine when driving onto a mound or suchlike. Owing to the high control rates of the hydrostatic unit required in combination with pause times to be maintained during a driving range change, new methods are needed in order to ensure the high level of dynamics required and also to protect the engine and the transmission.
DE 10 2013 215 520 A1 discloses a method for operating a power-split transmission device of a vehicle, whose transmission ratio can be varied continuously. The continuous adjustment of the transmission ratio in this case also takes place by way of a hydrostatic unit. A driving range change from a first driving range to a second driving range is carried out by switching over from a first range clutch associated with the first driving range to a second range clutch associated with the second driving range in accordance with a special control method which enables the driving range change to take place as comfortably as possible.
According to the generally known prior art, in a power-split transmission device the driving range change is carried out by means of range clutches, in that the range clutch for the new range is closed at the synchronous point or, usually, just before it. At the synchronous point the range clutch to be disengaged and the range clutch to be engaged no longer have a rotational speed difference between one another. When the new range clutch has been closed, the previous range clutch is opened again after a defined overlap time of the two clutches. During this overlap time the transmission ratio is at the synchronous point, so that during the overlap time the vehicle can only be decelerated by reducing the rotational speed of the engine.
The driving comfort and the productivity of a working machine, for example a wheel loader, are influenced essentially by the dynamics and quality of the driving range changes. In that regard it is vital for these to take place continuously and with a dynamic adapted to the driving situation. To determine the timing of the driving range change, it is usually calculated from the actual dynamic, the nominal dynamic and the distance of the transmission ratio at the time from the synchronous point, how long it is likely to be until the synchronous point is reached during a driving range change. Depending on the time interval so predicted, the new range clutch is pre-filled with hydraulic oil so that it can then be closed in accordance with a deceleration control logic that depends on the synchronous point. The synchronous point dependent deceleration control logic governs the further filling of the new range clutch in such manner that the mechanical loading of the clutch components is minimized and from the pre-filled condition the clutch can be closed as quickly and accurately as possible at the synchronous point. With the closing of the new range clutch the driving range change is active. If at that time the nominal transmission ratio has not yet reached the synchronous point, it is actively moved to the synchronous point. After that the transmission device can only produce lower or higher transmission ratios by means of the hydrostatic unit once the driving range change has been completed. The various driving range changes are linked to various gearwheel combinations in the transmission device. During the driving range change a defined overlap phase of the range clutches involved therein is maintained and only after the lapse of this overlap phase is the previous range clutch disengaged. Thereafter, the transmission ratio can be varied again.
For example, if the vehicle is to be decelerated, then in the normal case, i.e. in the absence of external disturbances, according to the sequence of the driving range change explained above, the change from a first, higher driving range FB2 with higher reciprocal transmission ratios to a second driving range FB1 with lower reciprocal transmission ratios is carried out by switching over from a correspondingly associated first range clutch to a second range clutch in accordance with the synchronous point dependent deceleration control logic. In this context reciprocal transmission ratio means the reciprocal of the value of the transmission ratio.
However, in a special situation such as when the vehicle drives up a mound, if an unexpected increase of the deceleration dynamic occurs at the beginning of the already initiated driving range change FBW from the first driving range FB2 to the second driving range FB1 then for the duration of this driving range change FBW the vehicle can only be decelerated by reducing the rotational speed of the drive engine. This may for example cause a Diesel engine to slow down to a sometimes unacceptable extent and, in the extreme case, to stall.
A further problem arises in that the time interval before the synchronous point is reached during the driving range change is calculated from the current actual or nominal dynamics. In doing this no account is taken of the fact that during the change the vehicle's dynamics could increase still further. In such a case the new range clutch would not yet be prepared in time by pre-filling and the driving range change FBW could only take place with some delay.